As a method of measuring concentration, etc., of a target gas contained in a measurement target gas, an absorption spectroscopic method is known. In an absorption spectroscopic method, measurement light in the absorption wavelength band of a target gas is irradiated to a measurement target gas, and the intensity of the transmitted light is measured. Then, the absorbance of the target gas is obtained from the light absorption amount by the target gas, and the concentration, etc., of the target gas is obtained from the absorbance.
Such an absorption spectroscopic method is used for measuring concentration, etc., of, for example, oxygen contained in a combustion gas supplied to an engine of an automobile, carbon dioxide contained in an exhaust gas from an engine. In this case, in order to perform the measurement in real time with the engine actually running, a probe is inserted into the combustion chamber of the engine or into the exhaust gas pipe, and the tip of the optical fiber is fixed at a predetermined position for the measurement. Such a probe is also used for measuring concentration, etc., of a gas in a flow path in a fuel cell or in a living body.
In Patent Document 1, a probe for optical measurement having a ferrule for fixing a tip of an optical fiber is described. In this probe for optical measurement, an optical fiber is inserted into a ferrule so that the tip of the optical fiber is taken out from the tip of the ferrule, and the periphery of the ferrule and the periphery of the portion of the optical fiber on the rear end side (the opposite side of the tip) than the ferrule are covered by a body sleeve formed of a metal tube. There is a gap between the optical fiber and the body sleeve on the rear end side than the ferrule, and a flexible protective tube is inserted in the gap. At the time of measurement, the tip of the probe for optical measurement is inserted into a hole provided in a combustion chamber of an engine, an exhaust gas pipe, etc., and the probe for optical measurement is fixed. With this, the tip of the optical fiber is fixed at a predetermined position in the combustion chamber or the exhaust gas pipe. According to this probe for optical measurement, the optical fiber is covered with the flexible protective tube on the rear end side than the ferrule. This prevents disconnection of the optical fiber due to bending.